Methods of counteracting gravitational forces on the human body have been devised for therapeutic applications as well as physical training. Rehabilitation from orthopedic injuries or neurological conditions often benefits from precision unweighting (i.e. partial weight bearing) therapy.
One way to unweight is to use a frame with elastic cords. Such existing systems are simple affairs, often relying on stretched bungee cords to provide the necessary unweighting forces. Use of bungee cords causes unweighting force to be poorly controlled, varying from cord to cord, over time, and with usage. In addition to a lack of repeatability, the inability to display unweighting force further prevents users from comparing current workouts with previous workouts. Furthermore, inability to easily adjust unweighting force requires user to dismount from the system to change settings. Frames are typically designed to be entered from the side, making close packing of systems over treadmills in a fitness club environment impractical. Also, these systems must typically be manually adjusted for differing user heights, complicating the usage process.
Another way to counteract the effects of gravity is to suspend a person using a body harness in conjunction with inelastic cords or straps to reduce ground impact forces. However, currently available harness systems are often uncomfortable and require suspension devices or systems that lift the user from above the user's torso. Such systems distribute weight unnaturally and uncomfortably on the user's body. The weight distribution can interfere with natural movements due to issues such as penduluming, quickly tightening/loosening, tilting the body, etc. In some cases, prolonged use with these harness suspension systems can result in injuries that range from mild skin abrasion or contusions or musculo-skeletal injury. In attempting to address the discomfort and limited mobility induced by such inelastic systems, some harness systems employ the use of bungee or elastic tensioning cords that need to be hooked and unhooked or manually stretched to adjust the degree of unweighting experienced. Such adjustment is cumbersome, inconvenient, and dangerous as the user may lose control of the tensioned cords during adjustment, causing the cords to strike the user with a substantial amount of force. All such overhead cord system do not constrain users from side-to-side or fore-and-aft motion, requiring users to focus on maintaining their position in space.
Other systems for unweighting a user have been developed. In one such system, a portion of a user's body is submerged into a water-based system to thereby permit buoyancy provided by the water offset gravity. However, both the upward supporting force and the effective point where the force is applied, provided by such water-based systems is dependent on the depth to which the user's body is submerged below the water surface, making unweighting force adjustability and natural weight distribution difficult to achieve, at best. Moreover, the viscous drag of the water may substantially alter the muscle activation patterns of the user. Users with open wounds, casts, splints, or other encumbrances are also not able to use water-based therapy.
Differential Air Pressure (DAP) systems have been developed to use air pressure in, for example, a sealed chamber to simulate a low gravity effect and support a patient at his center of gravity without the discomfort of harness systems or the inconvenience of water-based therapies. DAP systems generally utilize a chamber for applying differential air pressure to a portion of a user's body. While useful in training a wide variety of patient types, DAP systems have control systems to monitor and/or maintain pressure levels, pressure enclosures and the like to varying degrees based on the electrical and mechanical designs and complexity of the system, all of which add to the cost of such systems.
In view of the above shortcomings and complications in the existing unweighting systems, there remains a need for simple yet effective unweighting systems. In particular, for an average user who may not have a medical condition warranting physical therapy or medical supervision, there is also an additional need for unweighting systems suited to gym or home use. As such, a need exists for an unweighting system that allows users economical and effective alternatives to the current techniques available.
An important characteristic of unweighting systems intended for exercise or gait training is a low vertical spring rate, where the user's vertical position has minimal influence on the unweighting force applied to the user. This is significant because as a user walks or runs, their vertical displacement during different phases of the gait cycle can vary by ± two inches or more. A low vertical spring rate ensures that the unweighting force is nearly equal during all phases of the gait cycle. While fluid based systems such as DAP or pool-based therapies have inherently low vertical spring rates, the same is not true for most mechanical unweighting systems. The need for a low spring rate often requires the use of very long spring elements such as bungee cords, making these systems less than compact and/or unable to exert more than minimal unweighting forces. A further need is for a compact unweighting system with a low vertical spring rate.